Optical scanning device using plural sources of differing wavelengths

ABSTRACT

An optical scanning device having a first optical beam source for exciting a first optical beam to be modulated based on an image signal, a beam scanning device having a reflecting face for scanning the first optical beam on a photosensitive face, a second optical beam source for exciting a second optical beam for generating a modulation start signal for the first optical beam and a light receiving element for receiving the second optical beam having been reflected by the beam scanning device. The second optical beam source is arranged so as to prevent its reflected optical beam when scanned by the beam scanning device from being directed toward the photosensitive face. Having detected the second optical beam, the light receiving element generates a signal for starting the modulation of the first optical beam.

This is a division of application Ser. No. 027,750, filed Mar. 19, 1987now abandoned.

BACKGROUND OF THE INVENTION

(1) FIELD OF THE INVENTION

The present invention relates to an optical scanning device designedsuch that an image forming optical beam to form a latent image on aphotoreceptor member and a synchronous signal generating optical beamgenerated independently thereof are reflected on a same reflecting faceof a rotary polygon mirror.

(2) DESCRIPTION OF THE PRIOR ART

An optical scanning device employed in a laser beam printer and the likehas a rotary polygon mirror for scanning optical beam. A problem withsuch device is that lines to be scanned by the optical beam are notalways aligned with one another in a sub scanning direction because ofsome manufacturing errors in angle dividing precision of the reflectingface of the rotary polygon mirror or because of irregularities orvibrations associated with the rotation of the rotary polygon mirror.One prior art method attempting to overcome this problem is as follows:A light receiving section for detecting the optical beam is disposed ona scanning line more upstream than a photoreceptor member. When apredetermined time period has elapsed after the light receiving sectiondetects the optical beam, a modulation of this optical beam based on animage signal is started, thereby to synchronize a timing of the start ofthe modulation. Accordingly, the latent image formed by scanning by aplurality of times the optical beam on the photoreceptor member moved inthe sub scanning direction can be aligned with the sub scanningdirection.

However, it is necessary to dispose the above-described light receivingsection for detecting the optical beam and for controlling thesynchronization of the modulation so as to be away from the latent imageforming area on the photoreceptor member. In the case of a constructionusing the same optical beam both for forming the image and forsynchronizing the start timing of the modulation, it takes considerableamount of time for the optical beam to reach the latent image formingarea after being detected by the light receiving section. That is tosay, additional scanning of the optical beam is needed for generatingthe synchronous signal, thereby to slow down the image formingoperation.

One prior art method for overcoming this problem is disclosed inJapanese patent application laid open under NO. 53-3833, in whichindependent optical beams are used respectively for forming the imageand for generating the synchronous signal thereby eliminating theadditional scanning operation and consequently speeding up the imageforming operation. However, according to this method, in connection withthe rotation of the rotary polygon mirror, the optical beam forgenerating the synchronous signal scans also the latent image formingarea on the photoreceptor member. This results in the photoreceptormember also being exposed to the optical beam for generating thesynchronous signal, causing noises in image information leading to imagedeterioration. Therefore, there has been desired an optical scanningdevice with which the synchronous signal generating optical beam doesnot disadvantageously affect the image.

SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to overcome thedisadvantages in the prior art optical scanning device by providingimproved optical beam generating means for generating the synchronoussignal.

The forgoing object is accomplished in one embodiment by providing firstoptical beam generating means for generating a first beam to bemodulated based on an image signal, optical beam scanning means havingat least one reflecting face for scanning the first beam on aphotosensitive face, second optical beam generating means for generatinga second optical beam to be applied to the reflecting face, the secondoptical beam means being disposed in such a way as to prevent the secondoptical beam, to be scanned by the optical beam scanning means frombeing applied to the photosensitive face, and a light receiving elementfor detecting the second optical beam having been reflected by thereflecting face and for starting the modulation of the first opticalbeam thereafter.

More specifically, by causing the first beam, i.e. the optical beam forforming the image, to be reflected in an obliquely downward directionafter the beam is applied to the reflecting face from an obliquelyupward direction, or even in the case of scanning the two optical beamson the same plane, by causing both beams to be applied to the reflectingface of the rotary polygon mirror with suitable angle difference betweenthe beams, it is possible to refract the synchronous signal optical beamreflected by the reflecting face of the polygon mirror in a locationaway from the photoreceptor member, whereby the photoreceptor member isnot exposed to the synchronous signal generating optical beam and alatent image is formed without causing the noises in the imageinformation.

The foregoing object is also accomplished in another embodiment byproviding the first optical beam generating means for generating a firstbeam to be modulated based on an image signal, the optical beam scanningmeans having at least one reflecting face for scanning the first beam ona photosensitive face, the second optical beam generating means forgenerating a second optical beam to be applied to the reflecting face, awave length of the second beam being within a wave length range where asensitivity of the photosensitive face is substantially lower, and thelight receiving element for detecting the second optical beam havingbeen reflected by the reflecting face and for starting the modulation ofthe first optical beam thereafter.

According to the above construction, even if the second beam, i.e. thesynchronous signal generating beam is eradiated onto the latent imageforming area of the photoreceptor member, since a spectral sensitivityof the photoreceptor member is lower than the wave length of thesynchronous signal generating optical beam, the photoreceptor member isnot so considerably exposed as being exposed when scanned by the firstbeam, i.e. the image forming optical beam, whereby the latent image willbe formed which does not cause the noises in the image information.

The foregoing object is also accomplished in a further embodiment byproviding the first optical beam generating means for generating a firstbeam to be modulated based on an image signal, the optical beam scanningmeans having at least one reflecting face for scanning the first beam ona photosensitive face, the second optical beam generating means forgenerating a second optical beam to be applied to the reflecting face,the second beam having a lower beam intensity than the first beam, andthe light receiving element for detecting the second optical beam havingbeen reflected by the reflecting face and for starting the modulation ofthe first optical beam thereafter.

According to this construction, since the second beam, i.e. thesynchronous signal generating optical beam has the lower beam intensitythan the first beam, even if the synchronous signal generating opticalbeam is applied to the latent image forming area of the photoreceptormember, the photoreceptor member is not so considerably exposed by thissynchronous signal generating optical beam as being exposed when scannedby the image forming optical beam, whereby the latent image will beformed which does not cause the noises in the image information.

Especially, in the case the photoreceptor member is constituted by ahigh contrast material, i.e. the member has a large gamma value, it isreadily possible to render the intensity of the synchronous signalgenerating optical beam lower than a threshold sensitivity value of thephotosensitivity member merely by rendering the same about 1/10 lowerthan that of the image forming optical beam.

The foregoing object is also accomplished in yet another embodiment byproviding the first optical beam generating means for generating a firstbeam to be modulated based on an image signal, the optical beam scanningmeans having at least one reflecting face for scanning the first beam ona photosensitive face, the second optical beam generating means forgenerating a second optical beam to be applied to the reflecting face,the light receiving element for detecting the second optical beam havingbeen reflected by the reflecting face and for starting the modulation ofthe first optical beam thereafter and means for modulating the secondoptical beam so as to stop the second beam when the same is to bescanned on the photosensitive face.

According to this construction, since the synchronous signal generatingoptical beam is stopped by the modulating means therefore beforereaching the latent image forming area of the photoreceptor member, thephotoreceptor member is not exposed whereby the latent image will beformed which does not at all cause the noises in the image information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic construction view showing a first embodiment of anoptical scanning device related to the present invention,

FIG. 2 is a view showing transmission paths of optical beams in thefirst embodiment,

FIGS. 3 and 4 are views showing transmission paths of the optical beamsillustrating alternate embodiments of the first embodiment,

FIG. 5 is a schematic construction view showing second and thirdembodiments of the optical scanning device related to the presentinvention,

FIG. 6 is a schematic construction view showing a fourth embodiment ofthe optical scanning device related to the present invention,

FIG. 7 is a timing chart of the optical beams and of a synchronoussignal in the fourth embodiment, and

FIG. 8 is a graph showing spectral sensitivity characteristics of aphotoreceptor member and oscillation wave lengths of the optical beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will be particularlydescribed hereinafter with reference to the accompanying drawings.

FIG. 1 shows a schematic construction of a scanning device of a laserbeam printer by way of example of an optical scanning device related tothe present invention. A reference numeral 1 denotes an image formingsemiconductor laser device for exciting an image forming laser beam B(to be shortly referred to as image beam hereinafter) which is modulatedbased on input image information. The image beam B excited by this imageforming semiconductor device is processed through a collimator lens 2 toform a parallel light, which is then reflected by a reflecting face 3aof a high-speed rotary polygon mirror 3. With the rotation of thispolygon mirror 3, the reflected image beam B is scanned in alongitudinal direction of a photoreceptor drum 4 (this direction will bereferred to as main scanning direction).

This image beam B is projected by an f lens 5 to form an image on theuniformly electrified photoreceptor drum 4 thereby reducing anelectrostatic charge voltage at its present position. By repeating thisscanning operation with a constant speed rotation of the photoreceptordrum 4, an electrostatic latent image is formed on the photoreceptordrum 4.

Thereafter, though not shown, this electrostatic latent image undergoesa development, a transfer and a fixing and then is transformed into avisible image on a copy paper.

Independently of the image forming semiconductor laser device 1, thereis also provided a synchronous signal generating semiconductor laserdevice 6. This synchronous signal generating semiconductor device 6excites a synchronous signal generating laser beam B' (which will beshortly referred to as SOS beam hereinafter), which is processed throughanother collimator lens 7 and then is reflected by the reflecting face3a, which reflects the image beam B, concurrently therewith.

In the case of the present laser beam printer, the SOS beam B' is, asshown in FIG. 2, applied to the reflecting face 3a of the polygon mirror3 in an obliquely upward direction relative to a scanning field of theimage beam B and then is reflected in an obliquely downward direction soas to be directed toward a synchronization control light receivingsection 8. (It is to be noted here that the scanning field refers to anarea swept by scanning of the beam.) With this arrangement, even if theSOS beam B' is scanned with the rotation of the polygon mirror 3, theSOS beam B' does not reach a latent image forming area S of thephotoreceptor drum 4.

Therefore, on the photoreceptor drum 4 there is formed a latent imagewhich does not have noises in the image information resulting in animage quality deterioration.

The SOS beam B' is so arranged as to be scanned in an area including thesynchronization control light receiving section 8 with the rotation ofthe polygon mirror 3. This synchronization control light receivingsection 8 outputs a signal for adjusting a latent image forming positionfor each scanning operation relative to a rotational direction of thephotoreceptor drum 4 (will be referred to as sub scanning directionhereinafter).

Also, with the rotation of the polygon mirror 3, the SOS beam B' alongwith the image beam B is rotated in the same direction. The synchronoussignal generating semiconductor laser device 6 is positioned relative tothe image forming semiconductor laser device 1 in such a way as topermit the SOS beam B', which is scanned as well when the image beam Bhas reached a starting edge of the latent image forming area on thephotoreceptor drum 4, to reach the synchronization control lightreceiving section 8.

The signal output from the synchronization control light receivingsection 8 is input to a modulation control device 9, which outputs asynchronous signal SOS to a laser driving section 10 for the imageforming semiconductor laser device 1 when the output of thesynchronization control light receiving section 8 having received theSOS beam B' exceeds a predetermined value. Having received this signalSOS, the laser driving section 10 starts a direct modulation for theimage forming semiconductor laser 1 based on an image signal output froma memory 11, thereby starting an electrostatic latent image formingoperation using the image beam B which is modulated and output.

With the above-described construction, the electrostatic latent imageformed by repeated scanning operations on the photoreceptor drum 4 isaligned with the sub scanning direction. Further, since the synchronoussignal SOS is generated by using the SOS beam B' provided independentlyof the image beam B, the image beam B need be scanned over a smallestarea needed for the image formation, whereby the image formation may becarried out faster compared with the construction in which the imagebeam B is scanned as far as the synchronization control light receivingsection 8.

In the case of the above-described construction, the scanning field ofthe image beam B and that of the SOS beam B' cross each other. In placeof this, even if these scanning fields of the two beams B and B' are soarranged as to be positioned on a same plane, it is possible to preventthe SOS beam B' from being scanned in the latent image forming area ofthe photoreceptor drum 4 by suitably setting incidence angles of the twobeam B, B' onto the reflecting face 3a of the polygon mirror 3.

This will be described next with reference to FIGS. 3 and 4.

FIG. 3 illustrates a case where the image beam B and the SOS beam B'enter from a same side relative to a normal line N of the reflectingface 3a. At the center of the reflecting face 3a, if the incidenceangles of the image beam B and of the SOS beam B' are θ1 and θ2respectively, the beams B, B' having been reflected by the reflectingface 3a are refracted maintaining an angle of θ1-θ2.

This reflecting face 3a, with the rotation of the polygon mirror 3, hasan inclination thereof relative to either of the beams B or B' varied by2π/n (`n` is a number of the reflecting faces 3a constituting thepolygon mirror 3). Accordingly, both of the beams B and B' having beenreflected by the reflecting face 3a are refracted within ±2π/n frompositions thereof shown in the same figure, respectively.

That is to say, in order to prevent the SOS beam B' from being scannedover the latent image forming area S of the photoreceptor drum 4, it isnecessary for the scanning fields of the image beam B and of the SOSbeam B' not to overlap with each other. In other words, conditionssatisfying the following expression:

    θ2-θ1≧4π/n

are required.

FIG. 4 illustrates a case where the image beam B and the SOS beam B'enter from the opposite sides relative to the normal line N. In thiscase also, conditions satisfying the following expression:

    θ2-θ1≧2π/n

are required.

A second embodiment of the present invention will be described withreference to FIG. 5.

In the following description, it is to be noted, in order to avoidredundancy, only differences from the first embodiment will bediscussed. The rest are to be referred to the foregoing description ofthe first embodiment.

How the second embodiment differs from the first one in construction isthat the SOS beam B' excited by the synchronous signal generatingsemiconductor laser device 6, processed through the collimator lens 7and then reflected by the reflecting face 3a of the polygon mirror 3 isprojected by the f θ lens 5 and reaches the synchronization controllight receiving section 8 disposed nearer the starting point of scanningcycle than the photoreceptor drum 4.

In the case of the laser beam printer having the above-describedconstruction and constituting the second embodiment of the presentinvention, a wave length of the SOS beam B' is within a range lower orhigher than the spectral sensitivity of the photoreceptor drum 4. Thatis to say, with the rotation of the polygon mirror 3, the SOS beam B' isalso partially scanned on the photoreceptor drum 4. Thus, in order toprevent the latent image causing noises in the image information frombeing formed even if the SOS beam B' is scanned on the photoreceptordrum 4, the wave length of the SOS beam B' is out of the highsensitivity range of the photoreceptor drum 4.

More specifically, for example, in the case the image formingsemiconductor laser device 1 is constituted by GaAlAs which wave lengthvaries from 780 nm through 830 nm and the photoreceptor drum 4 includesa silicon compound which is highly sensitive in a range from 700 nmthrough 900 nm, the synchronous signal generating semiconductor laserdevice 6 may be constituted by InGaAsP for providing the SOS beam B'which wave length varies from 1300 nm through 1600 nm.

A third embodiment of the present invention will be particularlydescribed next.

This embodiment does not differ from the above-described secondembodiment in construction. However, while the wave length of the SOSbeam B' lies in the range where the spectral sensitivity of thephotoreceptor drum 4 is low in the second embodiment, in this thirdembodiment an intensity of the SOS beam B' is lower than that of theimage beam B and at the same time the same is lower than a thresholdphotosensitivity value of the photoreceptor drum 4. That is to say, ifthe SOS beam B' is scanned on the photoreceptor drum 4 with the rotationof the polygon mirror 3, the latent image is formed causing noises inthe image information. Therefore, in order to avoid this, even if theSOS beam B' is scanned on the photoreceptor drum 4, the above-describedarrangement prevents the photoreceptor drum 4 from being exposedthereto.

The intensity of the SOS beam B' need only be within a range where thenoises in the obtained image are practically negligible; thus, theintensity is not necessarily lower than the threshold photosensitivityvalue.

A fourth embodiment of the present invention will be described next.

In this embodiment, there is also provided an SOS beam modulation means12 for directly modulating the synchronous signal generatingsemiconductor laser device 6 in such a way as to permit the SOS beam B'to be scanned only in an area except the latent image forming area ofthe photoreceptor drum 4.

More particularly, if the SOS beam B' is scanned on the photoreceptordrum 4 with the rotation of the polygon mirror 3, the latent image isformed causing the noises in the image information. In order to avoidthis, the above arrangement prevents the SOS beam B' from being scannedon the photoreceptor drum 4. It is to be noted that clock signals formodulating the synchronous signal generating semiconductor laser device6 are provided by a drive clock (not shown) for the polygon mirror 3.

Also, it is possible to use the synchronous signal SOS output from thelight receiving section 8 as the signal for modulating the laser device6. In this case, a preset timer(s) is (are) started upon occurrence ofthe synchronous signal SOS thereby to stop the beam B' for apredetermined time period.

FIG. 7 is a timing chart illustrating the modulation of the image beam Bby the image forming laser device drive section 10, the modulation ofthe SOS beam B' by the SOS beam modulation means 12 and the synchronoussignal SOS.

As seen in FIG. 7, the SOS beam B' may be modulated by a relatively longcycle. This is because the SOS beam B' and the image beam B formdifferent angles with respect to the reflecting face 3a of the polygonmirror 3. Therefore, the SOS beam modulation means need not be of ahigh-performance type, whereby this construction may be simple andeconomical.

The SOS beam modulation means 12 may be of the afore-mentioned type forstopping the SOS beam B' for a predetermined time period; otherwise, themeans 12 may also be of a type for reducing the intensity of the SOSbeam B' while the same is being scanned on the photoreceptor drum 4 tobe lower than the sensitivity of the photoreceptor drum 4.

Nextly, alternate constructions and arrangements commonly applicable tothe above-described first through fourth embodiments will be describednext.

In the previous embodiments, the image beam B and the SOS beam B' areindependently provided by the two semiconductor laser devices 1 and 6,respectively. In place of this, it is also possible to obtain one or theboth of the beams from a gas laser device or from a solid laser device.

Further, in place of the aforementioned various types of laser beams, anoptical beam from an LED may be used for generating the synchronoussignal, these beams are generically referred to as the SOS beam B'.

In this case, in the fourth embodiment, if the SOS beam B' is providedby the gas laser device or by the solid laser device, an opticalmodulation device may be provided for modulating this SOS beam B'. Theoptical modulation device may comprise any of an AO (acoustic-optical)modulator, an electrochromic element, a PLZT element or even amechanical stop in case the modulation frequency is not considerablyhigh.

Also, in this case, in the second embodiment, the image beam, the SOSbeam and the photoreceptor drum are suitably selected referring to FIG.8 showing the spectral sensitivity characteristics of variousphotoreceptor drum 4 and oscillation wave lengths of various opticalbeams.

For instance, for the aforementioned combination of the image formingsemiconductor laser device 1 constituted by GaAlAs and the photoreceptordrum 4 constituted by a silicon, the SOS beam B' may be obtained from anHe-Cd laser device having a wave length of 442 nm. Further, of the laserbeams having two kinds of wave lengths obtained from an Ar laser device,one having a wave length of 514.5 nm may be used as the image beam Bwith a CdS photoreceptor drum 4 having a spectral sensitivity peakadjacent 520 nm and the other having a wave length of 488 nm may be usedas the SOS beam B'. In this case, it becomes possible to co-utilize apart of the optical system.

Still further, in the case the photoreceptor drum 4 is constituted by aselenium which is highly sensitive in a range from 450 nm through 650 nmand the image beam B is obtained from the Ar laser, it is possible touse as the SOS beam B' an optical beam from an LED having a wave lengthof approximately 700 nm.

Moreover, in the first, third and fourth embodiments, it is possible toobtain the SOS beam B' by separating the image beam B provided by thesame gas laser or solid laser device by means of a suitable opticalelement.

In this case, needless to say, such an additional arrangement is neededas disposing a filter in an optical path of the SOS beam B' for reducingthe light amount.

In all of the previously described embodiments, the image beam B and theSOS beam B' are applied to the same spot on the reflecting face 3a ofthe polygon mirror 3. In place of this, these two kinds of beams B andB' may be applied to different spots on the same reflecting face 3a.However, it is to be noted, in the case the reflecting face 3a of thepolygon mirror 3 does not have a high face-precision, it is better thatthe two kinds of beams B and B' are applied to the same spot.

The optical scanning device related to the present invention may beemployed not only for the laser beam printer described in theaforementioned embodiments, but also for a laser facsimile, a COM(Computer Output Microfilm) system in which a microfilm is exposed by alaser beam and the like.

In the case of the COM system and the like carrying out a high-densityrecording, a scanning lens such as the fθ lens 5 converges the imagebeam B into an extremely small radius. Therefore, in order to avoid aproblem that the excessively converged SOS beam B' may be hardlydetected by the synchronization control light receiving section 8, it isdesirable for the SOS beam B' not to pass the scanning lens, e.g. the fθlens 5. In this case, it is also to be noted, it is possible to form thescanning lens such as the fθ lens 5 compact since the same need onlycover the scanning range of the image beam B.

What is claimed is:
 1. An optical scanning device for scanning aphotosensitive member, comprising:first optical beam generating meansfor generating a first beam having a wavelength range within asensitivity range of the photosensitive member, the first beam beingmodulated based on an image signal and directed along a first opticalpath; beam scanning means having at least one reflecting face forscanning the first beam on a photosensitive face of the photosensitivemember; second optical beam generating means for generating a secondbeam to be applied to the reflecting face and directed along a secondoptical path separate from the first optical path, a design wave lengthof the second beam being within a wave length range where a sensitivityof the photosensitive face is substantially lower than the wavelength ofthe first beam; and a light receiving element for detecting the secondbeam having been reflected by the reflecting face and for outputting asignal for starting the modulation of the first beam.
 2. An opticalscanning device, as defined in claim 1, wherein said second optical beamgenerating means is disposed so as to permit the second beam toilluminate said light receiving element when the first beam reaches animage forming starting spot on the photosensitive face.
 3. An opticalscanning device for scanning a photosensitive member, comprising:firstoptical beam generating means for generating a first beam having awavelength range within a sensitivity range of the photosensitivemember, the first beam being modulated based on an image signal; beamscanning means having at least one reflecting face for scanning thefirst beam on a photosensitive face of the photosensitive member; secondoptical beam generating means for generating a second beam to be appliedto the reflecting face; said second beam having a significantly lowerbeam intensity than said first beam, the beam intensity beingapproximately lower than a threshold sensitivity level of thephotosensitive member; and a light receiving element for detecting thesecond beam having been reflected by the reflecting face and foroutputting a signal for starting the modulation of the first beam.
 4. Anoptical scanning device, as defined in claim 3, wherein said second beamgenerating means is disposed so as to permit the second beam toilluminate said light receiving element when the first beam reaches animage forming starting spot on the photosensitive face.
 5. An opticalscanning device, as defined in claim 3, wherein the beam intensity ofsaid second beam is approximately 1/10 of the beam intensity of saidfirst beam.
 6. An optical scanning device, comprising:first optical beamgenerating means for generating a first beam to be modulated based on animage signal; beam scanning means having at least one reflecting facefor scanning the first beam on a photosensitive face; second opticalbeam generating means for generating a second beam to be applied to thereflecting face; a light receiving element for detecting the second beamhaving been reflected by the reflecting face and for outputting a signalfor starting the modulation of the first beam; and second beammodulating means for modulating the second beam, said means stopping thesecond beam during the period that the second beam would be scanned onthe photosensitive face.
 7. An optical scanning device, as defined inclaim 6, wherein said second beam modulating means modulates the secondbeam in response to a signal synchronous with the scanning by said beamscanning means.
 8. An optical scanning device, as defined in claim 6,wherein said second beam modulating means modulates the second beam inresponse to the signal from said light receiving element.
 9. An opticalscanning device, as defined in claim 6, wherein said second optical beamgenerating means is disposed so as to permit the second beam toilluminate said light receiving element when the first beam reaches animage forming starting spot on the photosensitive face.